Note: When the complement is a noun, it must agree with the subject in number and gender.
e.g. Ahmed is their uncle. (not “uncles” or “aunt”)
e.g. Sarah and Huda are their aunts. (not “aunt” or “uncles”)
The full verb “BE” in this example only serves to join the subject and the complement together.
In this case, the full verb BE is called a Copula (=something that joins things together).
There are other copulas in English, for example, the verbs “become” and “seem